This invention relates to a process for reducing raw pellets containing metal oxides, such as iron oxide, zinc oxide, etc. to metallized pellets having a high metal content in a rotary kiln without forming powder of the pellets in the kiln or without ring formation in the kiln, and more particularly relates to a process for producing the metallized pellets by blowing air or spraying water into the portion near the charge end of the kiln in order to obtain metallized pellets.
It is desirable to recycle the iron-containing dust exhausted from blast furnace, converter furnace, electric furnace and open hearth furnace, etc. into the blast furnace in order to recover iron therefrom and to prevent environmental pollution. Furthermore, since the dust has about 50% by weight of iron, the reuse of the dust is desirable from the point of view of saving material resources.
The term "raw pellets" in the specification and the claims means pellets obtained by pelletizing the iron works dust exhausted from a blast furnace, electric furnace, converter furnace and/or open hearth furnace, etc. or a mixture of the dusts and optionally a carbon source, such as coal or coke. The term "iron-containing dust" in the specification and the claims means powdery materials withdrawn as by-products from blast furnace, electric furnace, converter furnace or open hearth furnace, etc. for smelting a metal or for making a metal. The term "metallized pellets" in the specification and the claims means pellets with a high concentration of iron obtained by chemically reducing said raw pellets.
It was known that pellets having a high metallic iron content can be prepared by reducing the raw pellets in a rotary kiln. One process for reducing the raw pellets in the kiln comprises the step of charging the pellets into the kiln, and the step of reducing the metal oxides contained in the raw pellets at a high temperature and at the same time volatilizing other metals, particularly zinc. The reduction of the metal oxides with carbon is represented by the following equation: EQU MO+C.fwdarw.M+CO
wherein M is a metal element.
Coal or coke may be mixed with the dust before pelletizing it. When the iron-containing dust has a high carbon content, the coal or coke may be unnecessary to be mixed with the dust.
The dust can be generally preheated at a temperature of 400.degree. C. or less so that carbon in pellets or metal oxide may not be burnt. Particularly, dust containing wustite or metallic iron can be preheated at a temperature only below 200.degree. C., because wustite or metallic iron is likely to be oxidized and subjected to exothermic reaction temperature above 200.degree. C. Therefore, preferably the iron-containing dust is preheated at a temperature below 200.degree. C.
One prior process for reducing the raw pellets to the metallized pellets has the following disadvantages: When the raw pellets are introduced into a rotary kiln, the raw pellets are ground during their passage through the kiln and as a result a ring is formed in the kiln. Therefore, the continuous operation in the rotary kiln is impossible. The cause of the ring formation has not been disclosed in the prior art.